1. Field of the Invention
The present invention relates to a method for manufacturing reduced iron agglomerates by reduction of iron oxide agglomerates incorporating carbonaceous material in a moving hearth reduction furnace, and also relates to an apparatus therefor.
2. Description of the Related Art
Recently, methods for manufacturing reduced iron agglomerates using inexpensive coal as a reducing agent instead of natural gas have again attracted attention. These methods are used to manufacture the reduced iron by heating pellets formed after blending powdered ore with carbonaceous material, such as coal, in a high temperature atmosphere in a reduction furnace.
The methods mentioned above will be described with reference to FIG. 8, which shows a rotary hearth reduction furnace disclosed in U.S. Pat. No. 5,730,775. Iron oxide agglomerates incorporating carbonaceous material are supplied from an inlet 56 and are then spread on the rotary hearth 51 of the reduction furnace so that the thickness of the agglomerate layer is approximately two agglomerates deep. Numeral 57 represents a leveler for leveling the thickness of the agglomerates. Agglomerates in approximately a double layer spread on the hearth move in a direction indicated by an arrow Y concomitant with rotation of the rotary hearth 51. The agglomerates are heated and reduced while being moved, and carbon dioxide and the like, generated when the agglomerates are being reduced, are discharged from a gas exhaust port 42. The reduced iron agglomerates yielded by the heating are discharged from the furnace by a discharging unit 54 after passing the leveler 60. The conventional technology described uses a reducing temperature between about 1,315.degree. C. and 1,430.degree. C., and a reducing time of approximately 10 minutes.
Powder containing iron oxide is also deposited on the surface of the rotary hearth 51, the powder being generated from the iron oxide agglomerates when the agglomerates are supplied into the reduction furnace. When the interior of the reduction furnace contains an atmosphere at high temperature, the iron oxide included in the deposited powder on the rotary hearth 51 is also reduced, and iron metal is therefore generated. When the reduction furnace is operated for a long period of time, the iron metal from the deposited powder which is reduced gradually accumulates so as to form a metal plate having a certain thickness, and the resulting metal plate causes problems in that the metal plate separates from the rotary hearth in the form of rolls or corrugated bodies. When the powder is being deposited, a problem may arise in that the hardened deposited powder having a certain thickness is separated in the form of blocks.
Types of seized hearth fragments are listed in FIG. 1. "Seized hearth A" seized hearth fragment is approximately 35 mm thick, 100 mm in width, and 150 mm in length and is in the form of a block. The "seized hearth A" tends to be generated when the reducing temperature is relatively low, and reduction of the iron oxide in the powder deposited on the hearth insufficiently occurs. Accordingly, the ratio of iron oxide (FeO) is high and degree of metallization is low. The reason for generation of "seized hearth A" is believed to be as follows. Gaps are generated between portions being metallized and portions not being metallized in the deposited powder, containing iron oxide, of a certain thickness, the gaps caused by thermal and mechanical stresses which are applied to the agglomerates during the cycle of supply, reduction with heat, and discharge thereof. Consequently, the hardened deposited powder is separated by a force applied thereto generated by the discharging unit. "Seized hearth C" seized hearth fragment is approximately 5 mm thick, 300 mm in width, and 2,000 mm in length in the form of a roll. The "seized hearth C" tends to be generated when the reducing temperature is relatively high, and reduction of the iron oxide in the powder deposited on the hearth occurs. Accordingly, the ratio of iron oxide (FeO) is low and degree of metallization is high.
FIG. 7 is a schematic view showing a state in which seized hearth fragment is generated in the form of a roll, that is, the so-called "seized hearth C". The powder containing iron oxide deposited on rotary hearth 51 is reduced at an elevated temperature in the reduction furnace, and forms deposited hearth 52. When deposited hearth 52 containing metallic iron increase to a certain thickness, this is separated from rotary hearth 51 by discharging unit 54 which discharges reduced iron agglomerates (pellets) 53 from the reduction furnace and then forms seized hearth fragment 55 in the form of a roll with reduced iron agglomerates rolled up therewith.
"Seized hearth B" seized hearth fragment is approximately 20 mm thick, 250 mm in width, and 300 mm in length in the form of a corrugated plate. "Seized hearth B" tends to be generated when the reducing temperature is medium.
"Seized hearth A" in the form of a block and "seized hearth B" in the form of a corrugated plate are discharged and recovered together with the iron reduced agglomerates from the reduction furnace. In this discharging step, "seized hearth A" and "seized hearth B" obstruct the product-recovery path for recovering the reduced iron agglomerate product, and problem occurs in that the operation of the reduction furnace may sometimes be interrupted. In addition, since seized hearth fragments having low degree of metallization are mixed with the reduced iron agglomerate product, the problem occurs in that the quality of the reduced iron agglomerates may be degraded.
In contrast, "seized hearth C" is so large that it cannot be discharged from the reduction furnace, and gradually grows in the shape of an enormous roll in proximity to discharging unit 54. Once this roll is formed, the reduced iron agglomerates are taken up in the roll and cannot be recovered. In addition, since the roll may damage the reduction furnace, problem may occur in that the operation of the reduction furnace must be terminated to remove "seized hearth C". Once the reduction furnace is stopped, a long period of time is required to restart the furnace, and frequent stopping of operation is therefore a very serious problem. In order to suppress generation of "seized hearth C", it is effective to lower the reducing temperature, as described above; however, metallization of product is reduced, and the quality of reduced iron agglomerates is therefore reduced.